This invention relates to optical position measurement systems, especially those used to measure the position of robot arms.
One known optical position measurement system uses a light source mounted on a mobile object cooperating with a fixed diffraction grating to produce a pattern of interference fringes. When the object, and thus the light source, move relative to the diffraction grating the fringe patterns will also move. By measuring the intensity of the fringe patterns using a photodetector fixed relative to the diffraction grating it is possible to calculate changes in the position of the object because, as the light source moves relative to the grating, the interference pattern will also move relative to the grating.
The main drawback of such systems is that the intensity measured by the photodetector can be altered for many reasons other than movement of the light source, such as changes in the various light generating and detecting elements and in the absorbtion and scattering of light between transmission and reception due to smoke or dust in the air.
This invention is intended to produce an optical position measurement system overcoming these problems.